The present invention relates generally to a collector of radiant energy of solar origin. A particular type of solar energy concentration and collection device is a cylindrical or trough shape collector comprised of an energy absorber and one or two opposing reflective side walls. The collector is formed by extending the transverse cross sections of the absorber and the reflector wall or walls longitudinally, parallel to an axis, to form the trough like structure. This is a non-imaging device and it has been determined that the optimal "ideal side wall countour" to achieve maximum concentration of energy incident on the entrance aperture onto the surface of the absorber is determined by the angle of extreme rays from the radiant energy source, the cross section of the absorber, and the desired maximum angle of incidence less than or equal to 90.degree. of concentrated energy on the surface of the absorber.
U.S. Pat. No. 3,923,381 describes the ideal side wall contour for concentrating energy from an infinite source onto the top surface of a flat energy absorber. U.S. Pat. No. 4,002,499 describeds the ideal side wall contour for concentrating energy from an infinite source onto non-flat energy absorbers and onto both sides of flat energy absorbers. U.S. Pat. No. 3,957,031 describes the ideal side wall contour for concentrating energy from a finite source onto an energy absorber of arbitrary shape. U.S. Patent Application Ser. No. 850,340, now U.S. Pat. No. 4,130,107, gives the ideal side wall contour for concentrating the energy from an infinite or finite source onto an energy absorber of arbitrary shape wherein the maximum angle of incidence of energy concentrated onto the surface of the energy absorber is restricted to a particular value less than 90.degree.. U.S. Patent Application Ser. No. 714,863, now U.S. Pat. No. 4,114,592, describes the ideal side wall contour for concentrating energy from an infinite or finite source onto an energy absorber of arbitrary shape wherein a refractive medium acts on the rays of incident energy prior to their being concentrated onto the energy absorber. A publication, Applied Optics, Vol. 15 No. 2, pages 291-292, February 1976 describes concentration of radiant energy from an infinite or finite source onto the surface of an arbitrary energy absorber wherein the reflective wall is obtained by total internal reflection within the interface between a dielectric medium and the exterior environment. To the extent, the above cited patents contain "essential" material necessary to support the claims hereof or indicate background they are expressly incorporated by references herein.
The characteristic of all these idealized non-imaging concentrators is the fact that the theoretical reflector side wall must touch the absorber surface. This is a theoretical requirement which is not met by reflectors actually used for solar concentration. In practice, the reflectors are isolated from the absorber by a gap which as a minimum provides an air gap to thermally isolate the absorber from the reflector. In many cases the gap must be large enough to accommodate a cover tube for a vacuum or a convection suppressing air gap, plus tolerances for tube essentricity, dimensional variation and bow and assembly tolerances. These gaps introduce losses of 10% or more in the efficiency of collectors. One solution or attempt to deal with the gap loss problem is described in U.S. Pat. No. 4,230,095 of Winston. Winston, in that patent, utilizes the concept of the virtual absorber. In actual practice, this design requires a space between the absorber tube and the reflector which can lead to gap losses of 10%. These losses arise even if the concentrator design is used not as a primary solar concentrator but as a secondary concentrator.
It is therefore an object of this invention to ideally eliminate gap losses associated with non-imaging trough shape solar collectors.
Another object of this invention is to provide a trough-shape, non-imaging solar collector having a portion of its side wall in the form of an ideal shape combined with a gap lossless reflector portion below the absorber.
Another object of the invention is to provide a solar collector having a gap lossless reflective wall positioned separated from an energy absorber for distributing incident energy onto the surface of the absorber.